People enjoy stereo sound systems for many purposes, such as for use with radios, television, movies, or other forms of entertainment or business involving musical or audio reproductions. Stereo sound systems are now even at times used with computers. A three-dimensional sound effect is desirable with stereo-sound systems, as it improves the sound impression and enhances the listener's experience by providing the listener with a sound impression that more closely approximates a live performance as opposed to a reproduction, especially as compared with two-dimensional sound.
Several methods of producing a three-dimensional sound effect are known which use four different channels with associated loudspeakers. For example, a method known under the trade name "Dolby Pro Logic" is currently used in many audio systems, including systems used with luxury television receivers. In such systems, a three-dimensional sound impression is conveyed by providing a right channel, a left channel, a center channel, and a rear channel. The rear channel may also be referred to as the surround channel. This four-channel distribution system provides a good three-dimensional sound effect, particularly for sound signals which seem to be coming predominantly from the center region located in front of the listener. In many cases, the stereophonic signals are not formed from genuine spatial signals, but rather, they are derived by providing different existing versions of a single audio signal to left and right speakers (or the left and right ears of a listener), via filter circuits. In that case, a pseudostereophonic effect is obtained, which nevertheless enhances the listening experience.
The use of four or more loudspeakers is frequently not possible due to a lack of space or cost of the speakers. Methods are known whereby different stereophonic signals can be combined via filter circuits to provide a satisfactory spatial effect with a reduced number of loudspeakers. In a certain sense, these methods represent a reversal of the above-mentioned pseudosystem.
An example of such methods is discussed in European Patent Application No. 94,305,664.8 (publication no. 0 637 191 A2), filed Jul. 29, 1994 by Lida Toshiyuki, et als., entitled SURROUND SIGNAL PROCESSING APPARATUS. The Toshiyuki application discloses a surround signal processing apparatus with which the number of sound reproducers can be reduced without losing the three-dimensional impression. Instead of using four speakers, a three-dimensional effect can be provided with a minimum of a right and a left speaker (or sound reproducers). The signals of the missing sound reproducers are electronically superimposed on the signals of the existing sound reproducers; the missing signal paths to the listener's right and left ears are electronically simulated via filter and delay circuits and the existing sound paths.
In simpler stereophonic sound systems, the center speaker, which represents a sound source located in front of the listener, is frequently eliminated by evenly dividing the center signal between the right and left channels already within the associated stereo filter circuit. This mode of operation is generally referred to as the phantom mode. The mode without a central sound reproducer is especially suited for television applications, since even luxury television sets generally have only two built-in speakers for the right and left channels. A separate speaker for the center channel can hardly be implemented for structural reasons.
The phantom mode (the division of the center signal between the right and left channels) is, in fact, often favored for television applications, in light of the relatively closely-spaced sound reproducers of a television set and the sound event itself. For example, televisions frequently show events on the center screen, such as a news announcer, a dialog scene, or a music group--the event thus often corresponds to the acoustic center position.
However, during television reception, while basically good sound impression is obtained when sound events are centralized, this contrasts with a poorer sound impression for events that are more distributed, such as particularly decentralized, sound sources. This poorer sound impression is due to the relatively small distance--the stereo base--between the two built-in speakers for the right and left channels. As a rule, the available stereo base width does not correspond with the viewing distance.
Circuits have been developed to address the reduced sound quality attributable to a small stereo base. For example, a circuit with which the stereo base can be widened is disclosed in co-pending U.S. application Ser. No. 08/754,144, filed Nov. 22, 1996, by inventor Winterer (the inventor herein), entitled SIGNAL MODIFICATION CIRCUIT, and assigned to Deutsche ITT Industries, GmbH (the assignee herein), which further claims foreign priority based on European Patent Application No. 95,118,595.8, and is incorporated herein by reference. In that invention, the right and left signals are modified by means of suitable filter circuits prior to loudspeaker reproduction. The differences in the signal waveforms above approximately 300 Hz in the right and left channels are enhanced and the common signal components are attenuated. The common signal components represent essentially a center signal.
Additionally, in the journal "ELRAD," 1994, No. 7, pages 76 to 81, analog circuits are disclosed with which the stereo base width of right and left signals is increased. Also known and described are circuits for creating spatial effects; these use output signals from commercially available stereo processors to produce specific spatial effects via external filter circuits.
The disadvantage of the stereophonic sound systems described above is that by applying the phantom mode--i.e., during electronic simulation of sound reproducers in the center position--they, to some extent, falsify the center impression through their filter circuits. If the stereo base width is also increased, the center impression is impaired even more.
It is therefore an object of the present invention to provide a circuit for use with a stereophonic sound system that conveys an improved three-dimensional sound impression, particularly an improved center impression, while using a reduced number of sound reproducers and a correspondingly small stereo base width.
It is a further object of the invention to provide a source of stereophonic signals that does not operate in the phantom mode with respect to its output signals, but remains in the normal mode.